Obesity is an epidemic with widereaching implications for health and society. Bariatric surgery is a successful therapy for achieving and maintaining weight loss and improving obesity comorbidiities. This study will explore the energetic mechanisms of weight loss in subjects undergoing gastric bypass bariatric surgery, which until now has not been clearly defined. In any scenario where there are substantial changes in energy stores, it is necessary to examine the role of changes in food intake and energy expenditure. After bariatric surgery, it is evident that energy intake declines. However, there is very little information as to how the components of energy expenditure, and in particular activity thermogenesis, change. This is not a trivial issue. In fact, changes in physical activity and activity thermogenesis could be crucial for understanding why some people lose more weight than others with bariatric surgery. Energy balance comprises energy intake and energy expenditure. Energy expenditure includes basal metabolic rate, thermic effect of food, and activity thermogenesis. Activity thermogenesis can be further divided into exercise activity thermogenesis and non-exercise activity thermogenesis (NEAT). The latter is even more difficult to measure than the former. Most Americans, and the vast majority of those with World Health Organization Class III obesity engage in relatively little purposeful exercise, and hence the majority of energy expended in physical activity is through NEAT. The aim of this study is to assess whether activity thermogenesis increases in patients undergoing bariatric surgery post-operatively compared with pre-operatively. We will assess NEAT using a validated Physical Activity Monitoring System and doubly labeled water, immediately pre-operatively and again at six and 18 months post-operatively. This is to assess energy expenditure, activity thermogenesis and NEAT during the phase of greatest weight loss and once weight loss has plateaued. This will permit us to address our primary hypothesis that activity thermogenesis as a fraction of total daily energy expenditure increases in patients undergoing bariatric surgery six and 12 months post operatively, compared to preoperatively. Overall, obesity is a major personal health and health care burden. Bariatric surgery has been shown to be beneficial without a clear understanding of the mechanism. The scientific need exists and the technology is available to uncover the role of activity thermogenesis and energy expenditure in weight loss after bariatric surgery. This may have dramatic implications for bariatric surgery and obesity management in general.